1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Wireless communication systems typically include one or more base stations or access points, which are part of the mobile network, for providing wireless connectivity to mobile units in a geographic area (such as a cell or sector) associated with each base station or access point. To initiate communication between a mobile unit and a base station, the mobile unit and the base station must establish a session. For this, the mobile unit attaches to the network, thereby creating in the network a “UE context” that includes information indicating values of various session parameters. UE context information may be transferred from one base station (or other network node) to another as the mobile unit moves between the cells, e.g., as part of a handover algorithm. Once a UE context has been established between the mobile unit and the base station, the mobile unit and the base station may form a wireless communication link, or air interface, which may be used to transmit modulated radiofrequency signals based on the session parameters. The air interface includes downlink (or forward link) channels for transmitting information from the base station to the mobile unit and uplink (or reverse link) channels for transmitting information from the mobile unit to the base station. The uplink and downlink channels are typically divided into data channels, random access channels, broadcast channels, paging channels, control channels, and the like.
Establishment of the communication session and/or wireless communication link is performed according to wireless communication standards and/or protocols. For example, members of the Third Generation Partnership Project (3GPP, 3GPP2) establish and agree to standards and/or protocols that may then be adopted by service providers for implementing their wireless communication systems. Adopting the agreed-upon standards and/or protocols permits service providers to cooperate in providing wireless communication service to subscribers. Examples of current and legacy wireless communication systems include General Packet Radio Source (GPRS) systems that operate according to the second and/or third generations (2G and 3G) of the 3GPP standards and/or protocols. The 3G systems may also be referred to as Universal Mobile Telecommunication Systems (UMTS). New standards and/or protocols, such as the Long Term Evolution/System Architecture Evolution (LTE/SAE) standards and/or protocols, are currently under discussion by the 3GPP.
Mobile units may have an active or an idle wireless communication link with one or more base stations. Active communication links are used when the mobile unit is actively transmitting and/or receiving information from the base station. When an active mobile unit moves, it may be handed off from a source base station to a target base station. Active mobile units may also be handed off from a source base station to a target base station when the quality of the wireless communication link to the source base station falls below the quality of a wireless communication link of a target base station due to changes in environmental conditions and/or transmission parameters of the source and/or target base stations. In the idle mode, a mobile unit may periodically reselect a preferred base station and synchronize to the corresponding broadcast paging interval. However, the idle mobile unit need not inform the new base station that it has been selected as the preferred base station. Thus, states associated with the mobile unit may not be moved until the mobile unit leaves the idle mode when forward link and/or reverse link traffic resumes.
When information becomes available for an idle mobile unit the wireless communication system transmits a paging message indicating that the information is available and requesting that the mobile unit enter the active mode to receive the information. However, as discussed above, idle mobile units are not required to notify base stations when they have been selected as the preferred base station. Consequently, the wireless communication system may not know the exact location of the idle mobile unit and therefore may be required to provide paging messages to numerous cells or sectors. Conventional wireless communication systems attempt to balance the competing desire to reduce overhead associated with transmitting location update messages and the desire to reduce overhead associated with transmitting paging messages by defining tracking areas that include a plurality of cells or sectors served by a plurality of base stations. Idle mobile units may then be required to transmit a location update message when it crosses the boundary between two tracking areas. Thus, the wireless communication system knows that the idle mobile unit is most likely within the tracking area indicated in the most recent location update message and so may begin the paging processed by providing paging messages via the base stations within the tracking area.
In the real world, wireless connectivity is provided to mobile units using multiple wireless communication systems, some of which may operate according to the most recent standards and/or protocols and some of which may operate according to one or more legacy standards and/or protocols. For example, the new LTE/SAE wireless communication system may be initially deployed by overlaying the system with existing legacy 2G/3G/UMTS wireless communication systems. The new and legacy wireless communication systems utilize different radio interface technologies that operate according to different standards and/or protocols. The legacy 2G/3G/UMTS wireless communication systems are expected to be, at least initially, more mature than the new LTE/SAE wireless communication systems and therefore the legacy wireless communication systems are expected to provide, at least initially, better and more reliable coverage. Consequently, mobile units are likely to utilize legacy wireless communication systems when coverage from the new wireless communication system is lost or not available.
Transitions between new and legacy wireless communication systems may generate a relatively large amount of signaling traffic, at least in part because both the new and legacy wireless communication systems may define overlapping sets of tracking areas. Consequently, the mobile unit may need to provide two location update messages every time it crosses a boundary between tracking areas associated with the new and/or legacy wireless communication systems. For example, if the mobile unit transitions from a tracking area associated with the new wireless communication system into a tracking area associated with the legacy wireless communication system, the mobile unit may provide location update messages to both the new and the legacy wireless communication system. The transitions may be the result of movement of the mobile unit, e.g., when a roaming mobile unit crosses a boundary between coverage areas of the new and legacy wireless communication systems, or the result of changing system or environmental conditions, e.g., when the channel quality provided by the new and/or legacy wireless communication system changes and triggers a transition between the new and legacy wireless communication systems.
The amount of signaling traffic generated by location update messages may also be particularly large when the new wireless communication system is first deployed because the coverage provided by the new wireless communication system may be patchy, resulting in relatively large numbers of transitions between the new and legacy wireless communication systems. The number of calls that are dropped by the new and/or legacy wireless communication systems may increase when the signaling traffic increases. Accordingly, the number of call drops may be undesirably large when the new wireless communication system is first deployed. Furthermore, a tracking area update triggered by a location update message that is transmitted by transitions between tracking areas maintained by different wireless communication systems results in the source wireless communication system releasing information (also known as a context) associated with the mobile unit. This context information must be re-created if the mobile unit transitions back to the original wireless communication system, which may lead to excessive resource consumption if the mobile unit frequently flip-flops between different wireless communication systems.